Developer



United States Patent DEVELOPER David A. Morgan, Roseville, Minn., assiguor to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware No Drawing. Filed Nov. 21, 1960, Ser. No. 7 0,431

10 Claims. (Cl. 20418) This invention relates to the formation of permanent visible reproductions of light-images on photoconductive surfaces by methods involving electrolysis at the exposed light-sensitive surface. It has particular reference to improved electrolytic developer solutions employed in such method. V v

This application is a continuationdn-part of U.S Serial No. 22,417, filed April 15, 1960, now abandoned.

Methods have recently been devised for electrolytically forming visible reproductions of light-images on strongly photoconductive light-sensitive copy-sheet materials. Apparatus for carrying out such methods, particularly in making enlarged reproductions of microfilm copies of graphic originals, has also been made available. Typically, there is involved the brief irradiation of a sensitive copy-paper with a light-image corresponding to the copy desired, and the subsequent electrodeposition at the conductive light-struck areas of a metallic image-forming material as the copy-paper is withdrawn from the irradiation area. The copy is obtained in substantially dry form, since only a surface application of developer solution is applied. No subsequent treatment, such as heating, washing, or subjecting to the action of chemical vapors, is required. Completed reproductions of microfilm transparencies or other graphic originals may be produced at a rate of ten or more per minute.

A recommended form of light-sensitive copy-paper for use with the apparatus and method hereinabove identified consists of a strongly-photoconductive zinc oxide surface coating on a metallized or metallic conductive backing or-sheet material. Such a sheet, having an 0.8 mil coating of four parts by weight of Zinc oxide and one part of polymeric insulating binder on a laminate of paper and aluminum foil, typically has a conductivity value, measured as hereinafter described, of at least about 10 mho/cm. and preferably not less than about 10* mho/cm. Under equilibrium dark conditions the conductivity value is not higher than about one-twentieth of the conductivity value under illumination, and ordinarily is much lower.

The conductivity value is measured as follows: A small sample of the sheet material is insulated at back and edge areas with a nonconductive waterproof covering, e.g., of plastic adhesive tape, an electrical connection to the conductive metallic substrate being provided. The sample issuspended in a transparent glass cell containing 200 ml. of a solution of tenth-molar ammonium sulfate and facing an open frame electrode serving as an anode. Current flow per unit area through the measured thickness of the coating under an applied voltage is measured both 'with the sample under equilibrium dark conditions and'when illuminated. A potential of 10 volts is convenient but not critical. Values at several thicknesses of coatings may be determined and the value at a standard thickness of 0.8 mil obtained by interpolation. Illumination is provided from a 500 watt incandescentfilament lamp, i.e. at an intensity of about1300 footcandles. The photoconductivityvalue is calculated from the values thus obtained.

As electrolytic developer solutions for use in the aboveidentified apparatus and method, there have been suggested solutions of salts of platable metals, e.g. copper sulfate, nickel chloride, or cadmium nitrate solutions. An initially visible image may indeed be formed with each of these; but for one reason or another the resulting copy has not been entirely satisfactory. For example, with platable base metals the image is found to fade rapidly when the copy is exposed to high humidity. Since some moisture is unavoidably present at the surface of the sheet following image development, conditions are normally unfavorable for image permanency. Furthermore, although electrolysis of many such salts does initially produce a visible image, the quality of the image is low. The image areas obtained are in most cases grayish rather than the desired dense black, and the copy lacks contrast. The noble metals produce no better, or even .less effective images. Silver nitrate, for example, produces a yellowish-brown image and a darkened background. Gold salts likewise produce images of low density. Solutions of gold and platinum salts are highly corrosive to metallic structural components and prohibitively expensive for most copying purposes. 7

Electrodeposition of metals from corresponding salt solutions, as in the electroplating art, is frequently improved in one respect or another by the inclusion of various additives in the plating solution. For example, soluble cyanides are normally added in small amountsto silver-plating baths to improve the brightness and permanence of the plate. It might therefore be expected that the substitution of typical metal-plating solutions for the simple salt solutions would likewise improve the qualiy of theimage obtainable in the electrolytic reproduction process hereinabove identified. It has been found, however, that electroplating formulations in general oifer inadequate improvement over the less complicated simple salt solutions in such process. Formulations employing base metals provide image areas of somewhat improved initial density and contrast, but the images are not permanent and fade or disappear rapidly when the sheet is held at high humidities. The cyanides are additionally highly undesirable because of their poisonous nature, since the residual unreacted salts, or residues resulting from the electrolytic decomposition of such salts, in all instances remain on the copy-paper rather than being removed, as in normal metal plating operations, by sub sequent washing.

On the other hand, additives which are employed to provide increased brightness of the metallic plate might appear likely to be disadvantageous in the electrolytic developmentof light-images on a white base, where a dense black rather than a bright metallic deposit is normally desired.

It has now been found that certain aqueous solutions of normally stable soluble electrolyzable complexes of platable metals to which has been added a water-soluble alkaline earth salt, such as magnesium acetate, calcium acetate and strontium acetate, provide electrolytic developer solutions with which by methods noted hereinbefore, may be obtained dense dark-colored image-forming deposits of a high degree of permanence, all as demonstrated by the following illustrative but non-limitative specific example.

Example 1 Parts by weight Silver nitrate "a 1.00

Thiourea 1.35 Glacial acetic acid (range 0.5 to 1.5)..- r 1 Magnesium acetate (range 4 to 15)" 5 Water to make 100.

The several components are mixed together at room temperature, forming a clear solution, which is placed in the metal reservoir of an electrolytic printing device. The sequence of admixing the components is exceptionally important in order to obtain a clear, colorless solution. The water and acetic acid should be admixed first, then the silver nitrate and thiourea are added in sequence.

slowly across the strip of sponge while the conductive.

backing of the sheet and the metal reservoir are connected to the poles of a battery. The sheet is thereby made the cathode of an electrolytic cell: As the sensitized sheet contacts thesolution in the sponge, a dense black deposit is formed at the conductive light-struck areas. The print shows no visible decreasein contrast or image of silver ion and ethylene thiourea (i.e. 2-imidazolidinethidensity after several Weeks in a high humidity test chamber 7 at normal room temperature.

Pressure between sheet and sponge, width of sponge face, and other factors areso controlled as to provide a minimum deposit of solution on the sheet consistent with the effective formation of the image. applied is insufficient to appear as a liquid layer on the sheet as it is taken from the printer. The moisture is soon volatilized,.the residual non-volatile material remaining on or in the oxide-coated surface.

When a solution of silver nitrate alone is employed in the foregoingpro'cedure as the electrolytic developer solution, the image areas are yellowish-brown in color C. The washed and dried crystals are found to melt.

at about 156 C. and to be substantially free of acetic acid, Dissolved in appropriate ionizable solvent, the pure crystalline complex likewise provides a stable liquid electrolytic developer solution useful in forming-permanent dense dark-colored image-forming deposits on photoconductive copy-paper.

In the absence of thiourea, the acetic acidvshown in The amount I Grams Silver nitrate 0.66

Ethyleneithiourea Q. 1.18

Acetic acid 1.00

Magnesium acetate a 5.00

Distilled Water V 92.16

V 4 agents, particularly thioureas, including the various members of the thiourea class, such-as ethylene thiourea (Z-imidazolidinethione), diphenyl thiourea, .etc., are ordinarily preferred. Complexes of other platable metals with these and other complexing agents, and. which are stable in solution form and on electrolysis produce permanent dark-colored, dense image-forming deposits, are also useful. i Although all members of the thiourea class form usable complexes with silver ion, some thiourea compoundsform a complex with silver ions that is even more stable than the thiourea-silver complex. For example, the complex one) provides an electrolytic developer with exceptional stability, as well as more stable image deposits and decreased pin-holing during electrolytiedevelopment. As with thiourea-silver complexes, a 3:1 mole ratiohas been found to be similarly criticalwith other members of the thiourea class. A typical'formulati'on providing excellent prints isas follows:

Any noble metal salt may be substituted for the silver nitrate with generally comparable results. Examples in- Example 1 is itself instrumental in increasing the density and improving the appearance of the image-forming deposits obtained with the silver nitrate developer solution. However, the complexes formed with this compound are relatively less stable and presumably are not present in the solution of the example, although such complexes may be present when less than three mols of thiourea are employed. The acetic acid is in any event helpful in forming the stable complex from silver nitrate and thiourea and, since it imparts no harmful effects, is'

ordinarily retained in the developer solution.

Maximum image density is obtained with the thioureasilver nitrate system at the three-to-one molar, ratio.

sulfide. The image areas therefore have much in common with images produced by silver halide photography; and the appearanceof the finished prints furthers this comparison. For this reason, stable soluble electrolyzable complexes of silver salts and sulfur-containing complexing clude gold chloride, silver chloride, silver acetate, platinum chloride, silver perchlorate, silver sulfate, palladium nitrate, and silver nitrate. Solublesilver salts are, however, preferred. Aqueous systemswill ordinarily be found preferable, but other ionizing solvents, are *known with whicheffective solutions of many of these. various complexes may be prepared. For example, glycerine 'and formamide may be used where the amount applied is so small in relation to the thickness. and absorbencyof the copy-paper as to avoid any necessity of subsequent solvent evaporation. I V

The acetic acid stabilizes the ,solutionto low 'temperatures and, in addition, when in combination with the magnesium salt, acts as a buffer (pH of about,5.2) during .the electrolysis. Thus, other. acid may be used in place of acetic acid (such as acetamide, which releases acetic acid) provided the anion is compatible-with the noble metal, i.e. does not produce an insoluble salt thereof.

The magnesium salt increases the rate of electrolysis 30 to 50 percent, thereby resulting in better contrast and a darkerwimage fo'r the same length of time of development. Also, the. magnesium salt preventspin-holing during electrolysis which sometimes occurs asthe result of voltage breakdown of copy-paper. Pin-holing is. characterized by dark spots or, heavy plating in the breakdown areas. a I

Having described my invention, .I claim:'

1. A stable liquid electrolytic developer solution having a pH below 7 which consists essentially of 'a dilute ionizablesolution of a stable complexof a silver salt and .a thioureain a 1:3 molar ratio, an acid and 4 to 15 weight athiourea in a 1:3 molar ratio, acid and 4 to 15 weight percent of a water-soluble alkaline earth salt buffering agent. 7 4. A stable liquid electrolytic developer solution having a pH below 7 which consists essentially of a dilute aqueoussolution of 1 to 10 percent by weight of a stable complex of a silver salt and a thiourea in a 1:3 molar ratio, 0.5 to 1.5 weight percent acetic acid and 4 to 15 weight percent of magnesium acetate.

5. A stable liquid electrolytic developer solution having a pH below 7 which consists essentially of a dilute aqueous solution of a stable complex of silver nitrate and thiourea in a 1:3 molar ratio, acetic acid and 4 to 15 weight percent of magnesium acetate.

6. A stable liquid electrolytic developer solution having a pH below 7 which consists essentially of a dilute aqueous solution of a stable complex of silver nitrate and ethylene thiourea in a 1:3 molar ratio, acetic acid and 4 to 15 weight percent of magnesium acetate.

7. A process for electrolytically reproducing an image which comprises exposing to a light-image a copy-sheet comprising a photoconductor bonded to a conductive carrier sheet, contacting and electrolytically developing said exposed sheet with an aqueous solution having a pH below 7 consisting essentially of a complex of a silver salt and a thiourea, an acid and 4 to 15 weight percent of an alkaline earth salt buifering agent, the mol ratio of silver salt to thiourea being 1:3.

8. A process for electrolytically reproducing an image which comprises exposing to a light-image a copy-sheet comprising a photoconductor bonded to a conductive carrier sheet, contacting and electrolytically developing said exposed sheet with an aqueous solution having a pH below 7 consisting essentially of a complex of a noble metal salt and a thiourea, acetic acid and 4 to 15 weight percent 5 of magnesium acetate, the mol ratio of metal salt to thiourea being 1:3.

9. The process of claim 8 in which said thiourea is ethylene thiourea.

10. The process of claim 8 in which said thiourea is thiourea.

References Cited in the file of this patent UNITED STATES PATENTS 1,903,860 Gockel Apr. 18, 1933 1,918,492 Schmidt July 18, 1933 2,040,142 Koreska et al. May 12, 1936 2,110,792 Egeberg et al Mar. 8, 1938 2,425,742 Kline Aug. 19, 1947 2,434,191 Benner et al Jan. 6, 1948 2,839,405 Jones June 17, 1958 2,849,351 Gundel et al. Aug. 26, 1958 3,010,883 Johnson et al. Nov. 23, 1961 3,072,541 Shely et a1 Jan. 8, 1963 FOREIGN PATENTS 151,971 Germany Dec. 8, 1902 OTHER REFERENCES Gray: Modern Electroplating, Wiley and Sons, New York, 1953, p. 371. 

7. A PROCESS FOR ELECTROLYTICALLY REPRODUCING AN IMAGE WHICH COMPRISES EXPOSING TO A LIGHT-IMAGE A COYP-SHEET COMPRISING A PHOTOCONDUCTOR BONDED TO A CONDUCTIVE CARRIER SHEET, CONTACTING AND ELECTROLYTICALLY DEVELOPING SAID EXPOSED SHEET WITH AN AQUEOUS SOLUTION HAVING A PH BELOW 7 CONSISTING ESSENTIALLY OF A COMPLEX OF A SILVER SALT AND A THIOUREA, AN ACID AND 4 TO 15 WEIGHT PERCENT OF AN ALKALINE EARTH SALT BUFFERING AGENT, THE MOL RATIO OF SILVER SALT TO THIOUREA BEING 1:3. 